It is widely known that any large drive system for use in, for example, a steel, paper, rubber or other processing mill requires precise speed control. Such precise speed control is dependent upon several factors, one being the integrity of the speed control device itself, another being the manner in which the speed control device is mounted on the drive system.
Heretofore, when it was desirable to measure the speed at which the output shaft of a drive system was operating, it was commonplcae to attach a speed control device or other indicator, such as a feedback tachometer or transducer, directly to the drive system output shaft. While this arrangement is capable of providing an indication of the speed at which the drive system is operating, unles the tachometer is perfectly centered on the output shaft, the tachometer will not provide a pure indication or signa of the true motor speed of the system. That is, although a tachometer that is disposed on the motor shaft off-center will provide an indication of motor speed, it may not be the true or exact motor speed in that unwanted error signals will also be indicated. The equation for tachometer feedback is expressed as follows: D.sub.FDBK =K.sub.N .times.RPM+K.sub.E sin .omega.t. The K.sub.N .times.RPM component is the ideal feedback signal which is the linear relationship between actual shaft speed, measured in revolutions per minute (RPM) and the feedback signal E.sub.FDBK. K.sub.N is a proportionality constant. K.sub.E sin .omega.t is a sinusoidal waveform with a magnitude factor of K.sub.E which is proportional to the amount of eccentricity between the motor shaft and the tachometer shaft. The frequency of oscillation is proportional to shaft speed. Typically with a drive motor turning 600 RPM, the frequency of oscillation is: ##EQU1## This frequency is passed through the tachometer amplifiers and cannot be filtered since the frequency is within the bandwidth of most high-performance control systems. This error signal ends up greatly amplified in motor armature current, setting up vibrational torque at or around the natural frequency of machinery causing large amounts of vibration, resulting in damage to both the product and the machinery producing it. In order to eliminate any unwanted error signals due to misalignment of the tachometer and to ensure a reading of the true motor speed, the tachometer must be aligned with the center of the motor shaft.
Centering devices have been previously suggested. Such devices have proven helpful in centering an attachment to a rotating element, and utilize an element adjustably received within a housing by opposing screws. Examples of such devices are found in U.S. Pat. No. 2,411,621 to Grubbs; U.S. Pat. No. 3,460,847 to Hohwart et al.: Italian Patent Document No. 505895; and Swiss Patent Document No. 393,499. However, none of these examples are directed to the particular environment of the present invention. That is, in which the rotating element is a motor shaft, and the housing is used to center a shaft on the motor shaft to which a measurement indicator is attached. Thus, until the development of the present invention, no one has provided a means by which a measurement indicator, such as a tachometer, can be easily secured to a motor shaft or other rotational device, but yet ensure that the measurement thus provided is consistent and accurate.